After sex my vagina got swollen and then the next day i had sex again and it start to burn like if got bruised in one side of my inner vagina lip. why?

Abrasion/Bruising: You may have an abrasion, essentially a friction burn. Also, with vigorous activity, you might have some bruising. Be sure you are well lubricated, either from foreplay or a water-soluble lubricant (e.g., KY jelly). Also, be sure your partner is gentle enough not to harm your fragile genital tissues. Finally, if you are sore "down there, " you might refrain from activity until you are fully healed.